In current radio communication systems, it is widely recognized that multi-input multi-output (MIMO) systems are a scheme effective in increasing throughput of a system. Among such MIMO systems, multiuser MIMO can more effectively increase system performance than single user MIMO. The multiuser MIMO is regarded as a further enhancement of the single user MIMO. In a successor version of the current Long-Term Evolution (LTE) system, studies are being carried out on the expansion of the current dual-layer beamforming (DL-BF) technique from single user to multiuser. In Rel-9, the multiuser MIMO can use a design similar to DL-BF of the single user MIMO. The multiuser MIMO is different from the single user MIMO in that data streams of different layers are transmitted to different users.
FIG. 1 shows a system that transmits two data streams to one user. As shown in FIG. 1, a plurality of antennas of cell base station 100 transmit two data streams after modulation, that is, a first layer data stream and a second layer data stream, to user 101.
FIG. 2 shows a system that transmits two data streams to two different users. As shown in FIG. 2, a plurality of antennas of cell base station 200 transmit two modulated data streams, that is, a first layer data stream and a second layer data stream, to two different users, that is, a first user and a second user. One outstanding feature of multiuser is that when a system scheduler and precoding vector are normally designed, intensities of data signals and reference signals (RS) of some layers are relatively high and intensities of data signals and reference signals of the other layers are relatively low on a receiving antenna of a certain user (e.g., 10 dB or lower).
The Rel-9 version defines that in single user DL-BF, reference signals are orthogonal to each other between two layers. Taking into account the fact that multiuser is an expansion of single user, reference signals of the plurality of users in a multiuser beamforming system must also be orthogonal to each other and an orthogonal scheme thereof may be time division multiplexing (TDM), frequency division multiplexing (FDM) or code division multiplexing (CDM).